Ozone that causes photochemical smog is created by photochemical reaction of HC and NOx contained in exhaust gases from automobiles and plants. Thus, reducing emissions of HC and NOx from automobiles is an effective means of holding down creation of ozone and thereby preventing occurrence of photochemical smog. Meanwhile, directly purifying ozone in the air can be another means of preventing occurrence of photochemical smog. Trying not only to reduce emissions of HC and NOx as reactants, but also to purify ozone as a product allows occurrence of photochemical smog to be prevented even more effectively. Against this background, in some areas including California, the U.S., automobiles including air purification systems for vehicles are put into practical use, the system being capable of directly purifying ozone in the air. Such air purification systems for vehicles are specifically called direct ozone reduction (DOR) systems.
The air purification systems for vehicles (DOR systems) that have conventionally been put into practical use incorporate a catalyst that is a metal oxide, such as manganese dioxide, as disclosed in JP-A-2002-514966 and JP-A-2003-515442. Application of a metal oxide catalyst to a radiator into which air flows during vehicle running allows the catalyst to decompose and purify ozone contained in the air.
Not only the manganese dioxide or other metal oxide catalyst, but also an activated carbon is known to have a function of purifying ozone. The ozone reacts with the activated carbon to become carbon dioxide. This reaction takes place even under room temperature, so that the activated carbon can be said to be advantageous in purification conditions, as compared with the metal oxide catalyst that requires a reaction temperature higher than the room temperature.
As of today, however, no air purification systems for vehicles, which use the activated carbon for an ozone purifier, have been put into practical use. Use of the activated carbon for the ozone purifier poses a problem in that purifying performance of the ozone purifier is easy to be degraded. The cause of this problem lies in the function of the activated carbon to adsorb moisture and PM, and NOx and SOx, contained in the air. These substances adsorbed on the activated carbon act as purification inhibitors inhibiting a reaction between the ozone and the activated carbon, seriously degrading the ozone purifying performance of the activated carbon. It is technically possible to recover the purifying performance of the activated carbon. That, however, involves complication of structures of the air purification systems for vehicles and requires a large amount of energy. In using the activated carbon in the air purification systems for vehicles, therefore, how to reduce effects of the purification inhibitors, such as moisture, NOx, SOx, and PM, on the activated carbon is a problem to be solved.